Check with Kettner's or Frankonia - I believe they have web sites. Handwarmer packets in the pockets and overboot covers with hand warmer packets inside, expedition weight underwear, Polartec fleece, and down. May take some experimenting, but warm is possible.
Be sensitive to the day when granddad just doesn't want to put up with any discomfort - even to hunt. In my case, it was a hard thing to understand that my quail huntin' buddy uncle had arrive at a point where shooting a shotgun, even from a dove stand, just wasn't any longer possible.
